"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Buick Somerset, if you buy parts for your car that are not actually made by Buick, then those are termed "aftermarket". Many people use the term "Somerset aftermarket parts" specifically when referring to custom parts for the Somerset, like performance parts or custom appearance parts. But even if you just replace your Somerset brake pads with OEM-grade factory-style brake pads that are not made by Buick, those too are aftermarket Somerset parts. Over the course of a vehicle's life, it almost certainly will have aftermarket parts installed in it at some point, even if the vehicle is never actually "modded out".